[NaCl SDK] Fix initialization of pthread_t in dlopen example

[NaCl SDK] Fix initialization of pthread_t in dlopen example

Initializing a pthread_t to NULL (or any value really)
is not valid.  This causes a compiler warning with the new
arm glibc toolchain so needs to be fixed because we
enable the new toolchain.

Turns out the pid_ member was not even needed so this
change removes it and adds a pthread_detach call which
allows for freeing on the thread's resources
(in the absence of pthread_join).

CQ_EXTRA_TRYBOTS=tryserver.chromium.linux:linux_nacl_sdk;tryserver.chromium.mac:mac_nacl_sdk
BUG=505885
Committed: https://crrev.com/b1904c3552fd0a572426430462c912efee2e0a98
Cr-Commit-Position: refs/heads/master@{#339720}

[Sam Clegg, 2015-07-21 17:04:28]

The CQ bit was checked by sbc@chromium.org to run a CQ dry run

[Sam Clegg, 2015-07-21 17:04:34]

sbc@chromium.org changed reviewers:
+ binji@chromium.org

[commit-bot: I haz the power, 2015-07-21 17:05:13]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1243123002/1

[binji, 2015-07-21 17:07:43]

lgtm

[Sam Clegg, 2015-07-21 18:23:32]

The CQ bit was unchecked by sbc@chromium.org

[Sam Clegg, 2015-07-21 18:23:45]

The CQ bit was checked by sbc@chromium.org

[Sam Clegg, 2015-07-21 18:23:49]

The CQ bit was checked by sbc@chromium.org

[commit-bot: I haz the power, 2015-07-21 18:24:58]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1243123002/1

[commit-bot: I haz the power, 2015-07-21 19:07:18]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-07-21 19:07:19]

Try jobs failed on following builders:
  win_nacl_sdk on tryserver.chromium.win (JOB_TIMED_OUT, no build URL)

[Sam Clegg, 2015-07-21 19:27:40]

The CQ bit was checked by sbc@chromium.org

[commit-bot: I haz the power, 2015-07-21 19:27:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1243123002/1

[commit-bot: I haz the power, 2015-07-21 19:36:42]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-21 19:49:21]

Patchset 1 (id:??) landed as
https://crrev.com/b1904c3552fd0a572426430462c912efee2e0a98
Cr-Commit-Position: refs/heads/master@{#339720}